The present invention relates to data transmission systems, and more particularly, concerns transmitting, receiving, and accessing information for XDSL and the like, transmission systems.
Telephone cables consist of a large number of twisted wire-pairs. Each twisted wire-pair conveys information to and from a particular customer. Digital Subscriber Line (DSL) technology is commonly used to allow for simultaneous voice and data traffic to coexist over a communication channel comprising a standard telephone transmission line. Typically, the standard telephone transmission line comprises an unshielded twisted-pair of copper wire having a gage of 22-26 AWG. Twisted pairs, which can be used to connect a central telephone system to a subscriber""s telephone system can support bandwidths up to 12 MHz through the use of digital signal processing (DSP) technology. Thus, they can be used for bandwidth-intensive applications, such as Internet access and video-on demand, as well as for carrying voice traffic. Frequency division multiplexing is used so that the plurality of signals, each occupying a different frequency band, can be simultaneously sent over the same transmission line.
Because there are different varieties of digital subscriber line technology, it is sometimes generally referred to as XDSL wherein the xe2x80x9cXxe2x80x9d represents a specific DSL standard such as HDSL for high bit rate digital subscriber line or RADSL for rate adaptive digital subscriber line, etc. As the name implies, ADSL is asymmetric in that the data transmission rates differ in the upstream and downstream direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office to a remote location that may be an end user such as a residence or business. The reverse signal corresponds to transmissions in an upstream direction, i.e., from the remote location to the central office. ADSL data traffic bandwidth for CAP (carrier-less amplitude and phase) modulation is typically from about 35 kHz to 1.1 MHz. The bandwidth for ADSL data traffic using DMT (discreet multi-tone modulation) is from approximately 26 kHz to 1.1 MHz. A separator is typically used in DSL systems to separate the upstream (transmit) and downstream (receive) signals on the twisted-pair transmission line.
High speed DSL transmission systems operating over twisted copper pair telephone lines are performance limited due to signal crosstalk between the wire pairs. The severity of the crosstalk increases with frequency. Additionally, as frequency increases, attenuation of transmitted signals also increases. Increases in crosstalk and attenuation of transmitted signals causes degradation of signal quality and restrictions on useful bandwidth for transmission. Restricting useful bandwidth further limits information capacity and speed of information transmission of telephone cables. Crosstalk is most severe within a central office where main feeder cables carry more wires than branch feeder or distribution cables. Hence, crosstalk is greatest at central offices and interferes more with weak signals entering offices than with stronger signals leaving offices. The result is that higher bit rates can be transmitted to users, referred to as downstream data transfer, than from users, referred to as upstream data transfer.
Other data transmission systems also exist, such as systems using an inverse multiplexed asynchronous transfer mode (IMA) that exhibit similar transmission limitations. IMA is one example among many of system methods used to divide information, on a signal to be transmitted, into multiple sections and therefore, multiple signals. In so doing, allowing for higher speed transmission, by transmitting the divided up sections on multiple twisted wire pairs and recombining the original information on a receiving end of the system. The receiving end contains formatting to recombine the originally transmitted signals. Data transmission systems using IMA and similar methods are limited by the number of wire pairs and time involved in dividing and recombining signals.
Existing telephone cabled systems, such as XDSL and systems utilizing IMA, employ two or more wire pairs to achieve a greater total information capacity. In these systems, each twisted wire pair conveys a unique portion of total information payload. The payload is sent on the twisted wire pairs within a main telephone cable. No attempt is performed, to improve or alter transmission of signals, on any other wire pairs within the telephone cable. Therefore, even though the information capacity is increased due to an increase in the number of wire pairs being used, crosstalk and attenuation of transmission signals remains a limiting factor in XDSL and IMA style systems.
Thus, there exists a need for a high speed data information transmission system that minimizes crosstalk and attenuation of transmitted signals, which provides for an increase in information capacity and transmission performance without an increase in the number of required twisted wire pairs.